BEIRUT, Jan 1 (Reuters) - Syrian warplanes resumed their bombardment of the rebel-held Wadi Barada valley northwest of Damascus on Sunday after nearly 24 hours with no air raids, a rebel official and the Syrian Observatory for Human Rights monitoring group said.

Rebels threatened on Saturday to abandon a Russian- and Turkish-backed ceasefire deal, which took effect on Friday, if the Syrian government and its allies continued to violate it.

The Observatory and a military news outlet run by Lebanese Hezbollah, a Damascus ally, said hundreds of civilians had left Wadi Barada in the last day for government-controlled areas nearby. (Reporting by John Davison; Editing by Mark Trevelyan)